A combined experimental and quantum chemical analysis of 3,3′-diaminodipropylamine–dihydrogenphosphate hybrid compound: [H3(C6H17N3)][(H2PO4)3]

Abstract An organoammonium-dihydrogenphosphate compound, [H3(DADP)][(H2PO4)3], DADP = 3,3′-diaminodipropylamine was synthesized and characterized. The crystal structure consists of a supramolecular organic-inorganic framework mediated via strong charge-assisted N–H···O and O–H···O hydrogen bonding, and non-classical C–H···O H-bonds. The NCI and QTAIM approaches revealed the occurrence of the noteworthy C–H···Csp3 carbon-centered H-bonds, and the C–H···H–C dispersive contacts. Intermolecular interactions were further elucidated by Hirshfeld surfaces analysis. Moreover, dispersion-corrected Density Functional Theory (wB97X-D/aug-cc-pVTZ) provided insights into chemical reactivity properties. The compound was also analyzed using solid-state spectroscopies and thermal experiments. This contribution enhances the understanding of the structural diversity of organic-dihydrogenphosphate compounds.


Introduction
Molecular crystals are crystalline solids consisting of two or more different molecular entities arranged in a periodic lattice structure and held together by intermolecular non-covalent interactions.The combination of organic amines with inorganic dihydrogenphosphate moieties has garnered significant interest due to the possibility of merging the structural and physicochemical properties of both components, leading to the formation of molecular crystals and salts with particular features [1][2][3][4][5][6][7][8][9][10].In organoammonium dihydrogenphosphates, the structural integrity of the crystals is primarily derived by a supramolecular hydrogen bonding network, particularly O-H … O and N-H … O hydrogen bonds.In addition, C-H … O hydrogen bonds have been observed, which play a supportive role in determining the crystal packing and overall stability [3,11].
DADP, or 3,3-diaminodipropylamine, is a significant organic amine molecule with diverse applications in multiple domains [12][13][14].DADP is a polyamine molecule with the chemical formula C 6 H 17 N 3 and a linear structure, consisting of two primary amine groups (-NH 2 ) and one secondary amine group (-NH-) connected by two methylene (-CH 2 -) linkers.One notable recent application of DADP is in rechargeable Zn-air batteries, where its addition prevents dendrite formation and improves cycle life and discharge capacity [13].In addition, it is used in the synthesis of amphiphilic PEECs, which are a drug/gene co-delivery vector with potential applications in drug and gene delivery [14].Moreover, Ren and coworkers utilized DADP in the in-situ assembly of ZIF-8 on nanopillars to create a self-adaptive surface that prevents bacterial infections and promotes tissue integration in biomaterial implants [12].Recent investigations have shown that dihydrogenphosphate-containing compounds exhibit promising potential for pharmaceutical and medicinal applications [15][16][17][18].For example, amifampridine phosphate drug has been used for treating Lambert-Eaton myasthenic syndrome (LEMS) [16].Furthermore, some medications used to treat basal cell carcinoma contain dihydrogen phosphate groups as a part of their chemical structure [15].In addition, the use of dihydrogen phosphates in drug products is not restricted due to safety concerns [17].
Based on the aforementioned facts, and as a continuation of our ongoing research on this topic [10], we present herein the synthesis and characterizations of a novel organoammonium-dihydrogenphosphate compound, [H 3 (DADP)][(H 2 PO 4 ) 3 ], DADP ¼ 3,3 0diaminodipropylamine.Crystals of the studied compound were prepared via slow evaporation method at room temperature and fully analyzed using single-crystal X-ray diffraction, thermal analyses (TG-DTA and DSC), and solid-state spectroscopies (NMR 31 P, 13 C, and IR).The supramolecular assembly has been analyzed through multiple computational tools i.e., Hirshfeld surfaces, 2D fingerprints, Atoms-in-Molecules (AIM) and Noncovalent Interactions Index (NCI).In addition, electronic properties and chemical reactivity have been evaluated through molecular electrostatic (MEP) and frontier molecular orbitals (FMOs) theoretical approaches based on Dispersion-corrected Density Functional Theory (DFT-D) at the wB97X-D/aug-cc-pVTZ level.

Chemical preparation and single-crystal X-ray diffraction
An aqueous solution of phosphoric acid (85%, d ¼ 1.7) was added to the organic molecule 3,3 0 -diaminodipropylamine (DADP) in the molar ratio 1:1.The resulting solution was slowly evaporated at room temperature for 3 weeks until colorless, parallelepipedic crystals of [H 3 (DADP)][(H 2 PO 4 ) 3 ] were formed.The structure was determined using single-crystal X-ray diffraction.Data were collected at room temperature using an Enraf-Nonius CAD4 diffractometer with Mo(Ka) radiation.The cell parameters were determined from a least square refinement of 25 reflections measured at high angles (1 -27 ).Two standard reflections were periodically measured every 2 h during data collection.3546 unique reflections were measured, only 2673 had their intensities (I > 2rI) and were used for structure determination and refinement.The crystal structure was solved using the direct methods with the program SHELX-97 from the WinGX package [19].All non-hydrogen atoms were first refined with isotropic and then with anisotropic displacement parameters.The final cycle of the refinement, including 635 parameters, leads to the reliability factors R 1 ¼ 2.11% and R w ¼ 4.09%.The crystal data and details of data collection and refinement are summarized in Table 1.

Physicochemical measurements
For detailed physicochemical measurements, please refer to our previous work [10].The NMR spectrum of a solid sample was obtained using a Bruker ASX300 spectrometer operating at 75.5 MHz for 13 C and 121.5 MHz for 31 P, with a spinning rate of 8 kHz and a pulse repetition time of 5 s.The IR spectrum was recorded at room temperature by making a thin and transparent pellet of the sample (2 mg) mixed with 100 mg of KBr and using a Biored FTS 6000 FTIR spectrometer with a resolution of about 4 cm -1 over the 4000-400 cm -1 wave number range.Thermal experiments were conducted using Setaram TG-DTA92 and DSC92 thermoanalyzers on 10.20 and 16.42 mg samples, respectively, by heating them at a rate of 5 C/min to 400 C under air.

Geometry optimization
All density functional theory (DFT) calculations were performed using Gaussian 09, Rev D.01 software [20].The structural model (Fig. 1) was constructed with GaussView 6.0 program [21], utilizing X-ray structure as a starting geometry, and then partially optimized at the wB97X-D/aug-cc-pVDZ level of theory.During the optimization process, all H and O atoms were optimized, while the positions of P, C, and N atoms were kept frozen.The long-range-corrected wB97X-D hybrid functional, which incorporates Grimme's D2 dispersion model, is considered well-suited for the analysis of noncovalent interactions [22].The validated frequency calculation confirms the stability of the optimized structure as the true minima by exhibiting absence of imaginary frequencies.This stable minimized model will be utilized as the initial geometry for all subsequent single-point calculations.

Single-point energy calculations
Topological analysis of the electron density (QTAIM and RDG-NCI) was carried out, from the DFT-based converged wave functions, employing Multiwfn 3.5 program [23], and corresponding graphs were drawn using and VMD 1.9.3 visualizer [24].The Molecular Electrostatic Potential (MEP) surfaces were generated and visualized at 0.004 a.u.isosurface.In addition, Frontier Molecular Orbitals (FMOs) were visualized using the Avogadro 1.2.0 molecular viewer [25].The MEP and FMOs surfaces were obtained from the minimized geometry of single-point calculations at a higher triple-zeta level of theory, wB97X-D/aug-cc-pVTZ.

Hirshfeld surfaces analysis
Hirshfeld surfaces were generated using the CrystalExplorer21 program [26].The d norm surfaces are mapped over a fix color scale of À0.25 au (red) to 0.80 au (blue).The shape index mapping range is À0.80 to 0.90 Å.The fingerprint cards were mapped at standard high quality (Isovalue ¼ 0.5) with expanded range 1-3 Å, and filtered by element.The normalized contact distance d norm combines both d i and d e , each normalized by the vdW radius r vdw of the corresponding two atoms involved in the close contact: where d i (d e ) designates the distance from a point on the surface to the nearest nucleus inside (outside) the surface.

Molecular structure and supramolecular features
The title compound crystallizes in the triclinic system, with P 1 space group, consisting of two crystallographically independent organic cations [H 3 (DAPA)] þ and six inorganic anions [H 2 PO 4 ] -in the asymmetric unit.Figure 2 illustrates the X-ray structure with As shown in Fig. 3a, each [H 2 PO 4 ] -group is interconnected with its analogous through four O-HÁÁÁO hydrogen bonds, resulting in the formation of an anionic supramolecular network.In addition, the DADP cations are linked to the anionic network through multiple N-HÁÁÁO hydrogen bonds (Fig. 3b).These organic cations act as linkers, facilitating the connection of eight anionic groups (Fig. 3c).Furthermore, nonclassical C-HÁÁÁO H-bonds were also observed, with C12 carbons acting as donors and O13 and O14 serving as acceptors, forming an alternate anionic-cationic infinite hydrogen-bonded chain (Fig. 3d).Table S3 in the supporting information provides a comprehensive list of D(donor) -HÁÁÁA(acceptor) hydrogen bonds, including those with H … A distances less than or equal to 2.12(2) Å and D-H … A bond angles greater than 129.3(1) .

Hirshfeld surfaces analysis
To visualize non-covalent intermolecular interactions and quantify their individual contributions to the crystal packing, the Hirshfeld surfaces approach was utilized as a powerful tool [27][28][29][30][31][32][33].First, two different surfaces were generated and mapped over d norm and d e properties using the.The intermolecular N-H … O hydrogen bonding between the anionic phosphate groups and organic moieties are clearly visible in Fig. 5 as several round red spots corresponding to various H … O interactions.Besides, twodimensional (2D) fingerprint cards were plotted and represented in Fig. 6.As anticipated, the significant contribution (62.2%) arises from H … O/O … H interactions which comprise more than half of the total Hirshfeld surface.The H … H close contacts have also important contribution of 37.7%.
We employed a combination of quantum theory of atoms in molecules (QTAIM) and the noncovalent interaction-reduced density gradient (NCI-RDG) approaches to identify and visually represent hydrogen bonding between organic and inorganic components.These methods were utilized extensively in our analyses of noncovalent interactions [28,31,34,35].The 3D color-filled RDG isosurface and 2D scatter plot for studied system are depicted in Fig. 7.As observed in the scatter maps, there are spikes (points that approach the bottom) in the negative region of XðrÞ (-0.05 to 0.0 a.u.), indicating that the structure exhibits evident attractive interactions.The presence of pure blue spikes in the very low-density   region of XðrÞ (À0.05 to À0.02 a.u.) clearly indicates the evidence of classical hydrogen bonds.The presence of green low-gradient peaks at X(r) % À0.018 a.u., confirms the existence of nonconventional hydrogen bonds and weaker attractive contacts.The QTAIM-NCI diagrams (Fig. 8) provide a clearer understanding of the type and nature of intermolecular interactions.Notably, the strong N-HÁÁÁO hydrogen bonds are prominently highlighted as blue-green NCI-isosurfaces (Fig. 8a).Similarly, the C-HÁÁÁO hydrogen bonds can be observed through the small green-colored isosurfaces, which appear between (-CH2) H-atoms and O acceptor atoms (Fig. 3b).These findings are consistent with the results of the X-ray structural analysis.Notably, the molecular graph in Fig. 4c illustrates the presence of a C-HÁÁÁC sp3 carbon-centered H-bond [36], indicated by relatively large green RDG isosurface, as well as bond critical point and bond path.In addition, the AIM-NCI graphs highlights the occurrence of C-H … H-C dispersive contact pairs between (-CH 2 ) groups of organic molecules [28,37] (Fig. 4d).

Chemical reactivity study: density functional theory calculations
Molecular Electrostatic Potential (MEP) is a concept used in computational chemistry to visualize the distribution of the electrostatic potential on the surface of a molecule.Figure 9 shows the MEP surface of the compound under study, which was generated at the DFT/wB97X-D/cc-pVTZ level using the optimized geometry.MEP outcomes are usually visualized using a color code, with red indicating areas of high electron density (i.e., negative potential), and blue indicating areas of low electron density (i.e., positive potential).The regions of high electron density are more likely to react with electrophiles, while regions of low electron density are more likely to react with nucleophiles.As depicted in Fig. 9 We have also conducted a FMOs analysis, Frontier Molecular Orbital, a computational method used to study the electronic structure and reactivity of molecules.It involves examining the energies and shapes of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of a molecule.Figure 10 presents the electron density distribution surfaces and energies of the frontier molecular orbitals for the title compound.The results indicate that the HOMO (1.989 eV) and LUMO (3.437 eV) are both localized in almost the same molecular regions.In FMOs theory, the HOMO is the electron donor orbital, and the LUMO is the electron acceptor orbital.The HOMO-LUMO gap represents their energy difference and determines the molecule's electron accepting or donating capacity.The small calculated HOMO-LUMO gap (1.448 eV) suggests that the studied compound is highly reactive, as it requires less energy to transfer an electron from the HOMO to the LUMO.This makes it more susceptible to oxidation, ionization, or chemical reactions.In addition, the small band gap implies that the compound has a tendency to absorb light at longer wavelengths.

Spectroscopic analysis
The 31 P NMR spectrum of crystalline monophosphate is displayed in Fig. 11a.It exhibits a central intense peak composed of three components, which are observed at 0.73, À1.82, and À3.87 ppm with three bands of symmetrical rotation.The chemical shift of these signals (-10 and þ5 ppm) corresponds to the expected value for phosphorus in tetrahedral coordination.The 13 C NMR spectrum is given in Fig. 11b.The organic molecule contains three different types of carbon atoms.The first type is represented by the C 1 methyl carbon atoms, which are bound to the nitrogen atom and give rise to a signal observed at 45.60 ppm.The second type is the C 3 carbon atom, which exhibits a chemical shift at 37.18 ppm.Finally, the third type of carbon atom is represented by the C 2 atom, which produces a less intense signal observed at 26.16 ppm.
The IR absorption spectrum in Fig. 12 shows the asymmetric and symmetric stretching vibrations of the PO 2 group at 1083 and 985 cm À1 , respectively, and two intense  bands corresponding to the bending modes of the P(OH) 2 group at 938 and 874 cm À1 .The 778 and 614 cm À1 bands correspond to the q(PO 2 ) rocking and d(OHPOH) bending vibrations, respectively.The 764 cm À1 band is attributed to the x(PO 2 ) wagging vibration.The two, strong bands at 522 and 502 cm À1 correspond to the torsion s(PO 2 ) and to the bending d(O-P-O) vibrations.Bands at 1352 and 1128 cm À1 are attributed to in-plane bending d(P-O-H), the out-of-plane bending vibrations g(P-O-H) are observed at 814 and 808 cm À1 .The IR spectrum displays a broad band ranging from 3595 to 2855 cm À1 , which can be attributed to the symmetric and asymmetric stretching modes of NH3 þ , NH2 þ , CH 2 , and OH.The bending modes of NH 3 þ , NH 2 þ and OH groups groups are observed as a strong band at 1655 cm À1 , a medium band at 1510 cm À1 and another medium band at 1500 cm À1 , respectively.

Thermal studies
The TG-DTA curves for the title compound are presented in Fig. 13a.The compound melts between 205 and 241 C, forming a viscous and transparent liquid upon further heating.The second thermal decomposition step involves the departure of ammonia from the structure and degradation, with endothermic effects observed at 211 and 266 C in the DTA curve.The DSC curve (Fig. 13b) shows a similar thermal behavior to the DTA thermogram.The endothermic peak observed at 204 C represents the melting of the compound (DH ¼ 26.07 KJ/mol).A range of endothermic effects between 220 and 370 C are attributed to the compound's decomposition and the elimination of ammonia.

Conclusions
In this study, we synthesized a novel organoammonium-dihydrogenphosphate hybrid compound, [H 3 (DADP)][(H 2 PO 4 ) 3 ], with DADP ¼ 3,3 0 -diaminodipropylamine.Through X-ray crystallographic analysis, we observed that the compound exhibits a welldefined supramolecular framework composed of both organic and inorganic components.The stability of this framework is largely attributed to the presence of strong charge-assisted N-HÁÁÁO and O-HÁÁÁO hydrogen bonds.In addition, we observed the existence of weaker non-classical C-HÁÁÁO hydrogen bonds, which further contribute to the overall stability of the structure.Using AIM and NCI computational approaches, we identified significant C-HÁÁÁC sp3 carbon-centered hydrogen bonds, as well as C-HÁÁÁH-C dispersive homopolar dihydrogen interactions between organic cations.The Hirshfeld surfaces revealed that HÁÁÁO and HÁÁÁH contacts are the primary contributors to the crystalline packing, accounting for 62.2% and 37.7%, respectively.DFT-based MEP calculations showed that the most negative regions correspond to negatively charged O-atoms, while the preferred nucleophilic attack regions are localized over the positively charged N-atoms of the organic cations.The compound's small HOMO-LUMO gap (1.448 eV) makes it highly reactive, susceptible to oxidation, ionization, and chemical reactions.Furthermore, solid-state spectroscopies (NMR 31P, 13C, and IR) and thermal measurements (TG-DTA and DSC) supported the X-ray findings and provide additional physicochemical properties.Our findings shed light on the structure and electronic properties of organoammonium-dihydrogenphosphate compounds, and may have implications for the design and synthesis of new materials with similar structural features.

Figure 1 .
Figure 1.Theoretical structure minimized using the dispersion-corrected DFT method at the wB97X-D/aug-cc-pVDZ level.Atom colors are: white (H), grey (C), blue (N), red (O) and orange (P).The labeling scheme is consistent with that in Fig. 2.

Figure 2 .
Figure 2. X-ray structure illustrating the asymmetric unit with labeling scheme and 50% probability ellipsoids.

Figure 3 .
Figure 3. Intermolecular hydrogen bonding motifs within the crystal structure: (a) O-H … O hydrogen bonds between dihydrogen phosphate groups.(b) and (c) N-H … O hydrogen bonding.(d) Non-classical C-H … O H-bonds.H-bonds are depicted by dashed green lines, with donor and acceptor contact atoms represented as spheres.

Figure 5 .
Figure 5. Three-dimensional (3D) Hirshfeld surfaces, mapped with d norm (left) and d e (right) properties.Intermolecular N-H … O hydrogen bonds are shown as green dashed lines.

Figure 10 .
Figure 10.Electron density distribution and energies of frontier molecular orbitals calculated at DFT/wB97X-D/cc-pVDZ level of theory.